This invention relates to implantable cardiac pacemakers, and particularly to rate-responsive cardiac pacemakers. More particularly, this invention relates to a system and method for modulating the base rate by a transfer function for a rate-responsive pacemaker, between a resting rate that is suitable for the patient while awake but at rest, and a sleeping rate that meets the patient's low metabolic demands during sleep.
A pacemaker is an implantable medical device that delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate--typically a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, although in more advanced pacemakers, the rate could be set remotely after implantation. Such pacemakers were known as "asynchronous" pacemakers because they did not synchronize pacing pulses with natural cardiac activity.
Early advances in pacemaker technology included the ability to sense the patient's natural cardiac rhythm (i.e., the patient's intracardiac electrogram, or "IEGM"). This led to the development of "demand pacemakers"--so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, hemodynamically effective cardiac contraction which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding contraction. When a naturally occurring contraction is detected within the escape interval, the demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable because pacing pulse inhibition extends battery life and avoids competition with the patient's intrinsic rhythm.
Modern demand pacemakers allow physicians to telemetrically adjust the length of the escape interval, which has the effect of altering the heart rate maintained by the device. However, in early devices, this flexibility only allowed for adjustments to a fixed programmed rate, and did not accommodate patients who required increased or decreased heart rates to meet changing physiological requirements during periods of elevated or reduced physical activity. Therefore, unlike a person with a properly functioning heart, a patient receiving therapy from an early demand pacemaker was paced at a constant heart rate--regardless of the level to which the patient was engaged in physical activity. Thus, during periods of elevated physical activity, the patient was subject to adverse physiological consequences, including lightheadedness and episodes of fainting, because the heart rate was forced by the pacemaker to remain constant.
The adverse effects of constant rate pacing lead to the development of "rate-responsive pacemakers" which can automatically adjust the patient's heart rate in accordance with metabolic demands. An implanted rate-responsive pacemaker typically operates to maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increases the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Rate-responsive pacemakers typically include processing circuitry that correlates measured physical activity to an appropriate heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and a slope defining transition rates between the minimum heart rate and the maximum heart rate, are parameters that may be telemetrically adjusted to meet the needs of a particular patient.
One approach that has been considered for enabling rate-responsive pacemakers to determine an appropriate heart rate involves the use of a physiological parameter that reflects the patient's level of metabolic need. Physiological parameters that have been considered include central venous blood temperature, blood pH level, QT time interval and respiration rate. However, certain drawbacks (such as slow response time, unpredictable emotionally-induced variations, and wide variability across individuals) render the use of these physiological parameters difficult, and accordingly, they have not been widely used in practice.
Rather, most rate-responsive pacemakers employ sensors that transduce mechanical forces associated with physical activity--the level of physical activity being indicative of the patient's level of metabolic need. These activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. By analyzing the signal from a piezoelectric activity sensor, a rate-responsive pacemaker can determine how frequently pacing pulses should be applied to the patient's heart.
Piezoelectric elements for activity sensors are commonly formed from piezoelectric crystals, such as quartz or barium titanite. Recently, however, activity sensors have been designed which use thin films of a piezoelectric polymer, such as polyvinylidene fluoride (commonly known by the trademark KYNAR, owned by ATOCHEM North America), rather than the more commonly used piezoelectric crystals. Activity sensors so designed are described in copending, commonly-assigned U.S. patent applications Ser. No. 08/059,698, filed May 10, 1993, now U.S. Pat. No. 5,383,473 entitled "A Rate-Responsive Implantable Stimulation Device Having a Miniature Hybrid-Mountable Accelerometer-Based and Method of Fabrication," and Ser. No. 08/091,850, filed Jul. 14 1993, now U.S. Pat. No. 5,425,750 entitled "Accelerometer-Based Multi-Axis Physical Activity Sensor for a Rate-Responsive Pacemaker and Method of Fabrication," which are hereby incorporated by reference in their entireties.
A variety of signal processing techniques have been used to process the raw sensor signals provided by activity sensors. For example, in one approach, the raw signals are rectified and filtered. Alternatively, the frequency at which the highest peaks in the signals occur can be monitored. Regardless of the particular method used, the result is typically a digital signal that is indicative of the level of sensed activity at a given time. In one preferred approach, the digital signal is produced by repeatedly integrating the raw sensor signals until a predetermined threshold value is reached. Each time the threshold is reached, a digital trigger pulse is generated. A counter is used to count the number of trigger pulses that occur in a fixed period of time (e.g., the number of trigger pulses that occur during an approximately 100 ms period within each heartbeat interval). The count reached at the end of the fixed period of time is provided to processing circuitry in the pacemaker, which processing circuitry typically includes a microprocessor.
The processing circuitry then uses the count signal to produce an activity level measurement that represents the patient's activity level. The appropriate rate at which the patient's heart is to be stimulated (known as the sensor-indicated rate) is determined by applying a transfer function to the activity level measurement. The transfer function defines a sensor-indicated rate for each possible activity level measurement.
An example of a rate-responsive pacemaker in which a transfer function is used to calculate the sensor-indicated rate is described in commonly-assigned U.S. Pat. No. 5,074,302 of Poore et al. ("the '302 patent"), which is hereby incorporated by reference in its entirety. As described therein, when relatively little activity is detected, the activity level measurement is ordinarily below a low activity threshold. When the activity level measurement is below the low activity threshold, the sensor-indicated rate is set to a base pacing rate (e.g., 60 beats per minute (bpm)), as defined by the transfer function. At high levels of measured activity, the activity level measurement may exceed a high activity threshold. When this occurs, the sensor-indicated rate is limited to a maximum pacing rate, so that the patient's heart is not stimulated too rapidly. If the value of the activity level measurement falls between the low and high activity thresholds, the pacemaker applies pacing pulses to the patient's heart in accordance with the rate determined by the transfer function, generally at a rate somewhere between the base pacing rate and the maximum pacing rate.
Typically, for activity level measurements between the low and high thresholds, the transfer function is linear. The slope of the transfer function determines increases (or decreases) in the pacing rate corresponding to a given increase (or decrease) in the activity level measurement. The larger the slope, the more rapidly the pacing rate will increase (or decrease).
The slope of the transfer function in typical rate-responsive pacemakers is telemetrically adjustable by a physician, so that the operation of a pacemaker can be tailored to suit an individual patient's needs. During follow-up visits, the slope may be adjusted by the physician if the patient's condition warrants a change. However, for some patients, more frequent slope adjustments may be desirable. In view of this need, pacemakers have been designed which can automatically adjust the slope of the transfer function. The '302 patent describes one such approach--in which high and low averages of activity sensor readings are used in connection with preprogrammed base and maximum pacing rates to derive an appropriate slope for the transfer function.
Another approach for automatically adjusting the slope of the transfer function is described in commonly-assigned, copending U.S. patent application Ser. No. 08/255,194, filed Jun. 7, 1994, entitled "System and Method for Automatically Determining the Slope of a Transfer Function for a Rate-Responsive Cardiac Pacemaker," which is hereby incorporated by reference in its entirety. The approach described therein uses the patient's activity profile, as represented by an activity level histogram stored in the pacemaker's memory, to adjust the slope of the transfer function. The activity level histogram collects activity level measurements over a predetermined period of time, preferably about a week. Each week, the activity level histogram is evaluated to determine if a slope adjustment is warranted. If an adjustment is deemed to be appropriate, the activity level histogram is used, in connection with preprogrammed base and maximum pacing rates, to define the new slope. The activity level histogram is then cleared so that new data may be collected for the next adjustment cycle. In addition, the pacemaker described in copending U.S. patent application Ser. No. 08/255,194, filed Jun. 7, 1994, advantageously inhibits slope adjustment if it is determined that the patient was bedridden for a significant portion the most recent data collection cycle (i.e., the previous week). Further, the above copending U.S. patent application, Ser. No. 08/255,194, filed Jun. 7, 1994, describes an approach that can be used to determine a slope that accommodates a patient's regular exercise routine.
The advances described in the '302 patent and the above copending application Ser. No. 08/255,194, filed Jun. 7, 1994, have lead to the development of extremely flexible pacemakers that enable bradycardia patients to achieve a level of cardiac performance that closely resembles that of healthy individuals. However, there are certain areas in which flexibility can be improved even further. For example, in most rate-responsive pacemakers, the base pacing rate (which, as described above, defines the minimum heart rate maintained by the pacemaker) is usually set telemetrically by the physician in connection with the implantation procedure and then afterward, as needed, during follow-up visits. The base pacing rate is usually set at a rate that comfortably meets the patient's metabolic needs for when the patient is awake but relatively inactive.
While a healthy individual is awake but relatively inactive, the individual's heart rate is usually maintained at a "resting rate." During sleep, the heart rate of a healthy individual typically drops to a "sleeping rate" that is below the resting rate. In this respect, pacemaker-assisted cardiac performance usually differs from what is ordinarily experienced by healthy individuals. More precisely, the fixed base pacing rate of the pacemaker (which is analogous to a healthy individual's resting rate) prevents the patient from experiencing a sleeping rate, which if available, may be more comfortable for the patient during sleep.
The difference between the sleeping rate and the resting rate for healthy individuals is usually rather small (typically in the range from about 10 bpm to about 20 bpm). However, the inability of some pacemakers to maintain a sleeping rate may cause the patient to have some difficulty falling asleep, and may occasionally lead to a restless night of sleep. In addition, since it is likely that a sleeping pacemaker patient being paced at a resting rate could withstand (and even benefit from) a lower sleeping rate, the pacemaker wastes limited energy reserves by maintaining the unnecessarily high resting rate.
In view of the foregoing, it would be desirable if the base pacing rate of a rate-responsive pacemaker could be modulated between a resting rate that is suitable for the patient while awake but relatively inactive, and a sleeping rate that meets the patient's low metabolic demands during sleep. It would also be desirable if the base pacing rate could gradually transition between a sleeping rate and a resting rate, so that abrupt rate changes as the patient transitions between sleep and wakefulness can be avoided.